The present invention relates to a magnetic recording medium having a ferromagnetic metal layer as a magnetic recording layer disposed on a stretched polyethylene-2,6-naphthalene multilayered film. More particularly the present invention relates to the magnetic recording medium which is high in output, and excellent in output stability and running property.
Further improvement of magnetic recording density strongly required in the art, and recently a non-binder-type magnetic recording medium has become into the spotlight in place of the conventional coating type magnetic recording medium in which the magnetic material is dispersed in an organic binder and coated and dried, or a magnetic recording medium in which the binder in the dispersion is cross-linked by electron radiation. This new magnetic recording medium is characterized by forming a thin film of a ferromagnetic metal on the surface of a non-magnetic support such as polyester film or polyimide film by a suitable method such as vacuum deposition method, sputtering method, ion plating method, electroplating method, electroless plating method, etc. As is well known, for the increase of recording density, it is an essential requirement to make the magnetic layer as thin as possible in view of the thickness loss. In the non-binder-type magnetic recording medium, it is required that the thickness of the magnetic layer is usually not more than 1 .mu.m, preferably not more than 0.5 .mu.m.
As the thickness of the magnetic layer is reduced, the unevenness of the support surface becomes more liable to affect to the surface of the magnetic layer, resulting in noticeably affecting the performances of video film and audio films, for example, electromagnetic properties such as output, output variation, output defect, etc. of the video films or audio films as compared with the conventional magnetic recording media.
Therefore, in the development of magnetic recording media having such a ferromagnetic metal layer, smoothness of the support is required more than ever.
However, as the film surface is smooth, the slippery property of the film reduces, resulting in deteriorating its handling property and running property. If the running surface of a magnetic recording medium, that is, the film surface on the side opposite from the ferromagnetic metal layer is smooth, running of the tape becomes unstable and, in an extreme case, the tape becomes unable to run.
For a magnetic recording medium provided with a ferromagnetic metal layer satisfying both requirements for smoothness and slipperiness, there are proposed to dispose a coating layer on the base film (Japanese Patent Publication (Kokoku) Nos. 55-47050, 60-50150, 63-28097, or to use a base film wherein the roughness of both sides thereof is different each other (Japanese Patent Publication (Kokoku) No. 55-15770). It was also proposed to use these techniques in combination (Japanese Patent Publication (Kokoku) Nos. 1-26337, 1-26338).
Employment of these known techniques is still insufficient to well meet the recent strict requirements for the magnetic recording media. Further, when a magnetic recording medium is wound up, there often takes place backside transfer phenomenon that roughness on the opposite side transfers to the surface of the magnetic layer, whereby it is difficult to attain desired high quality.
Polyethylene terephthalate film has been popularly used as base film for magnetic recording media, but the polyethylene-2,6-naphthalate film (hereinafter referred to as PEN film) with which it is relatively easy to attain increase of recording density, is more suited as base film for the magnetic recording media having a ferromagnetic metal layer, of which further reduction of thickness is required. However, the adhesion between such PEN film and the ferromagnetic metal layer formed by vacuum deposition method, is not always satisfactory.
As a result of earnest studies for overcoming the said prior art problems, it has been found that by forming a coating layer (A) having characteristics defined by the following formulae (1)-(3) concerning a three-dimensional average protuberance height "Ha" (unit: .mu.m), the number of the protuberances "Na" (unit: protuberances/mm.sup.2) and its hardness "HD" (unit .mu.m.sup.-2) in its surface, on the high-roughness surface of a stretched polyethylene-2,6-naphthalate multilayered film, then forming a coating layer (B) having specificities defined by the following formula (4) concerning a three-dimensional average protuberance height "Hb" (unit: .mu.m) in its surface on the low-roughness surface of the said multilayered film and further forming a thin ferromagnetic metal film on the said coating layer (B), the obtained magnetic recording medium has substantially no scratches on its ferromagnetic metal film surface by protuberances of the multilayered film, shows a stabilized running property, is capable of maintaining the VTR head output almost unchanged from its initial level after 100 times of repeated runs, and thus is high in output and excellent in output stability. EQU 0.01.ltoreq.Ha.ltoreq.0.08 (1) EQU 10,000.ltoreq.Na.ltoreq.60,000 (2) EQU 10.ltoreq.HD.ltoreq.150 (3) EQU 0.001.ltoreq.Hb.ltoreq.0.008 (4)
The present invention was achieved on the basis of this finding.